The present invention relates to a process for removing soot from the exhaust gas of a diesel engine by oxidizing the nitrogen monoxide present in the exhaust gas to nitrogen dioxide, separating the soot form the exhaust gas stream and oxidizing the soot by using the nitrogen dioxide used. The invention can be used to particular advantage with diesel engines in the lower power range of up to 100 kW.
The limiting values planned for the year 2005 in the European Union for diesel vehicles are aimed at the simultaneous reduction of nitrogen oxide and soot emissions. The limiting values planned for vehicles with a registered total weight of less than 2.5 tons are 0.25 g/km for nitrogen oxides (NOx) and 0.025 g/km for particles. Constructional measures in diesel engines can only reduce one of these two harmful substances at a time, while more of the other is simultaneously produced.
Thus, for example, the emissions of soot are reduced by optimizing the combustion process. The relatively high combustion temperatures required for this, however, lead to increased production of nitrogen oxides. The amounts of nitrogen oxides in their turn can be reduced by measures such as exhaust gas recirculation (EGR), but this again increases the emissions of particles. The current status of development of diesel engines for the field of application mentioned represents an optimum with respect to the emission of nitrogen oxides from these engines in the partial load region is less than 100 vol.ppm; the emission of particles is about 0.5 g/km. Attempts to reduce one of the two components further, by actions taken in the engine, automatically lead to increased emissions of the other hazardous component.
Therefore, the planned exhaust gas limiting values can only be complied with by means of suitable post-treatment of the exhaust gas. The main focus of efforts is to reduce the emission of soot, since the emissions of nitrogen oxides from modern diesel engines are already very close to the planned limiting values.
Currently, soot filters are mainly used for the efficient reduction of soot emissions. So-called wall-flow filters are widely used for this purpose, these being constructed in a honeycomb manner, in a similar way to known catalyst support structures. In wall-flow filters, alternate ends of the flow channels for the exhaust gas passing through the honeycomb structure are sealed so that the exhaust gas has to flow through the porous walls in the filter. These types of wall-flow filters filter up to 95% of the soot from the exhaust gas.
The back pressure of the filter increases as the soot loading increases. Therefore the filters have to be regenerated by burning off the soot. Current conventional methods for soot combustion use thermal or catalytic processes or else additives to the fuel for the continuous or cyclic regeneration of particle filters. Temperatures higher than 600xc2x0 C. are required for the thermal combustion of soot. The ignition temperature of soot can be lowered considerably by a catalytic coating on the filter.
Thus, DE 3407172 C2 describes a device for treating the exhaust gases from diesel engines which contains a sequence of filter elements either lightly packed or displaced from each other in a housing, wherein at least one filter element A supports a coating which lowers the ignition temperature of soot and at least one filter element B supports a catalyst which promotes the combustion of gaseous hazardous substances and filter elements A and B alternate with each other several times.
EP 341832 B1 describes a process for treating the exhaust gas from heavy trucks. The exhaust gas is first passed over a catalyst, without filters, in order to oxidize the nitrogen monoxide contained therein to nitrogen dioxide. The exhaust gas, containing nitrogen dioxide, is then used to burn off the particles deposited on a downstream filter, wherein the amount of nitrogen dioxide is sufficient to bring about the combustion of particles stored on the filter at a temperature of less than 400xc2x0 C.
EP 835684 A2 describes a process for treating the exhaust gas from vans and passenger cars. According to this process, the exhaust gas is passed over two catalysts connected in sequence, the first of which oxidizes nitrogen monoxide present in the exhaust gas to nitrogen dioxide which then oxidizes soot particles, which have been deposited on the second catalyst, to CO2, wherein nitrogen dioxide is reduced to nitrogen in accordance with the following equation:
xe2x80x832NO2+2Cxe2x86x922CO2+N2xe2x80x83xe2x80x83(1)
WO 99/09307 describes a process for reducing soot emissions from heavy trucks. According to this process, the exhaust gas is first passed over a catalyst to oxidize nitrogen monoxide to nitrogen dioxide and then over a soot filter on which the deposited soot is continuously oxidized. Some of the purified exhaust gas is passed over a cooler and then mixed with the intake air for the diesel engine.
A prerequisite for the trouble-free functioning of the last process is the presence of a sufficient quantity of nitrogen dioxide for as complete combustion as possible of the soot contained in the exhaust gas to proceed in accordance with equation (1).
Modem diesel engines for vans and passenger cars, however, generally use exhaust gas recirculation (EGR) in order to keep the nitrogen oxide emissions as low as possible. In the exhaust gas from these types of diesel engines, less than 100 vol.ppm of nitrogen oxides are generally observed. As a result of the low exhaust gas temperatures ( less than 250xc2x0 C.), in particular from passenger car diesel engines, the efficiency of the process is further restricted because, even with high nitrogen oxide emissions, sufficient amounts of nitrogen dioxide cannot be produced.
The amounts of nitrogen oxides emitted by modern diesel engines with exhaust gas recirculation is not generally sufficient to ensure complete oxidation of soot under all running conditions. In addition, the inventors could not establish complete nitrogen oxide reduction in accordance with equation (1) in the partial load region for passenger cars with diesel engines. Almost as large a quantity of nitrogen oxides was emitted from an exhaust gas treatment device in accordance with EP 835684 A2 as entered the device.
An object of the present invention is therefore to provide a process which also uses oxidation of soot by means of nitrogen dioxide, but which can also be used in the types of diesel engines which have only a low initial emission of nitrogen oxides due to the way the engine is operated. Another object of the invention is to create a device for performing the process.
The above and other objects are achieved by a process for the removal of soot from the exhaust gas from a diesel engine by oxidizing the nitrogen monoxide present in the exhaust gas to nitrogen dioxide, separating the soot from the exhaust gas stream and oxidizing the soot by using the nitrogen dioxide produced. The process is characterized in that the particular procedure described is performed in at least two consecutive process stages and the soot is separated from the exhaust gas stream in each process stage with an efficiency W of between 0.05 and 0.95, wherein each process stage can be allocated a transmission for soot (T) in accordance with T=1xe2x88x92W and the total transmission of the process for soot is given as a product of the transmissions for all the process stages.
The efficiency W of soot separation, in the context of this invention, is understood to be the difference between the weight of soot per unit of time entering a process stage and the weight of soot per unit of time passing through the process stage, divided by the weight of soot per unit of time entering the process stage. Accordingly, the transmission of a process stage for soot is the quotient of the weight of soot per unit of time passing through a process stage and the weight of soot entering the process stage. The efficiency W of soot separation and the soot transmission T for a process stage are related to each other via T=1xe2x88x92W.
The total transmission Tg of all the process stages is given by the product of the transmissions Ti of all the process stages as expressed by the following equation:                               T          g                =                              ∏            l            n                    ⁢                      xe2x80x83                    ⁢                      T            i                                              (        2        )            
Tg: Total transmission
Ti: Transmission of ith process stage
n: Number of process stages
The present invention is based on the observation that the oxidation of diesel soot in the partial load range of diesel engines largely proceeds not in accordance with equation (1) but more probably in accordance with the following reaction scheme:
NO2+Cxe2x86x92CO+NOxe2x80x83xe2x80x83(3)
and
2CO+O2xe2x86x922CO2,xe2x80x83xe2x80x83(4)
since, despite a reduction in the amount of soot separated, no substantial conversion of nitrogen oxides can be measured.
Nitrogen dioxide is thus largely reduced only to nitrogen monoxide under exhaust gas conditions, such as exist under partial load for diesel engines, for oxidizing soot particles in accordance with equation (3).
Since the low concentration of nitrogen oxides in the exhaust gas from modern diesel engines is not sufficient to oxidize all the particles emitted, separation of the soot particles according to the invention is therefore divided into several process stages, wherein the efficiency of separation is between 0.05 and 0.95.
The number of process stages to be used depends on what fraction of the soot emitted can be oxidized by the nitrogen oxides present in the exhaust gas. Since the ratio of nitrogen oxides to soot is not a constant for the engine, but depends, on the particular running conditions, fixing the number of process stages is a compromise. 2 to 4 process stages are generally sufficient.